One apparatus for measuring the permeability of a porous sheet material is described in U.S. Pat. No. 2,861,451, issued Nov. 25, 1958, to N. Emmons. In Emmons, a sample of the porous material is clamped into the tester so that one surface of the material is supported on a perforated planar surface. Air under pressure is caused to flow through the porous material, then through the perforated planar surface, and is then exhausted to the atmosphere after flowing through a tapered, transparent float tube containing an indicating float which positions itself along the tube in accordance with the velocity of air flowing through the tube. The position of the float provides an indication of the permeability of the porous material. The requirement that each sample to be measured must be clamped in the tester makes the apparatus described in the Emmons patent unsuitable for continuously measuring the permeability of a traveling web.
Another instrument capable of measuring the permeability of a sample of a porous web is the Gurley permeability tester. The Gurley permeability tester utilizes a bell or cylinder of aluminum operating in a seal of mineral oil so that as it descends, it forces air through a specially mounted specimen of paper. The volume of air displaced through the specimen is measured by the amount the bell sinks. The usual test consists of determining the time in seconds required to displace 0.0001 cubic meters of air through 0.000645 square meters (1.0 square inch) of paper under a pressure head fixed by the weight and dimensions of the bell. The result obtained, which is inversely related to air permeability, is referred to as the air resistance.
An apparatus capable of continuously measuring the permeability of a moving porous web is described in U.S. Pat. No. 3,371,518, issued Mar. 5, 1968, to M. A. Keyes. In Keyes, the web travels over a hollow, perforated, rotating roll. A stationary suction box within the roll establishes an air pressure differential across the traveling web. In one embodiment, the interior of the suction box is maintained at a predetermined pressure and the flow of air through the suction box is measured and provides an indication of the permeability of the web. In another embodiment, the air flow rate to the suction box is kept constant and the pressure within the suction box is measured to provide an indication of the permeability of the web. One disadvantage of the apparatus described in the Keyes patent is the difficulty of providing a good seal between the rotating roll and the stationary suction box. It is also believed to be a disadvantage to locate the pressure measurement or control point outside of the suction box where it measures pressure changes due to inadequate sealing between the rotating roll and the stationary suction box and due to the expansion of the air as it enters the suction box from the apertures in the rotating roll.
Another apparatus capable of continuously measuring the permeability of a traveling porous web is disclosed in U.S. Pat. No. 3,466,925, issued Sept. 16, 1969, to P. D. Ziegenhagen et al. In that patent, a measuring head is located so that it contacts the moving web. A vacuum is applied to the measuring head which draws air at a constant flow rate through the paper and into the measuring head. The vacuum pressure under the head is measured and provides an indication of the permeability of the web passing across the head. The Ziegenhagen et al. patent also discloses that the measuring head can include a plurality of sealing apertures connected to a vacuum source that is separate from the measurement vacuum source. This sealing section prevents air from flowing under the moving web and into the measuring section of the head.
Both Keyes and Ziegenhagen et al. measure a pressure within the apparatus that includes the pressure differential across the web. In both cases the pressure measurement is taken at a location that is relatively remote from the web, that is, after the air has passed through the apertures of the measurement surface. As a result, the pressure measurement not only measures the pressure changes due to the varying permeability of the web but also the pressure changes due to the expansion of the air as it leaves the apertures of the measuring surface and the pressure changes due to inefficient sealing of the vacuum source, etc. If these latter two parameters are not constant, the pressure measurement will not accurately represent the permeability of the web.